The invention relates to clamping apparatus and methods for fuel cells.
Fuel cells have been used to produce electrical power. A fuel cell is an electrochemical device that converts energy produced by a chemical reaction into electrical energy. Fuel cells generally employ an ion exchange membrane or solid polymer electrolyte disposed between two electrodes that form the anode and cathode. One type of fuel cell includes a proton exchange membrane (PEM) fuel cell. At the anode of the PEM fuel cell, diatomic hydrogen (a fuel) is oxidized to produce hydrogen protons that pass through the membrane. The electrons produced by this oxidation travel through electrical circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water.
Multiple fuel cells can be connected together, generally in series, to increase the voltage output of the fuel cell assembly. Several serially connected fuel cells may be formed in an arrangement called a fuel cell stack. The fuel cell stack may include different plates that are stacked one on top of the other in the appropriate order, and each plate may be associated with more than one fuel cell of the stack. The plates may be formed of metal or a graphite composite material and may include various channels and orifices to route the above-described reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells.
Referring to FIG. 1, as an example, a fuel cell stack 10 may be formed out of repeating units called plate modules 12. Each plate module 12 includes a set of composite plates that may form several fuel cells. For the arrangement depicted in FIG. 1, an exemplary plate module 12a may include a cathode cooler plate 14, a bipolar plate 16, a cathode cooler plate 18, an anode cooler plate 20, a bipolar plate 22 and an anode cooler plate 24 that are stacked from bottom to top in the listed order. Each cooler plate acts as a heat exchanger by routing a coolant through flow channels in either the upper or lower surface of the cooler plate to remove heat from the plate module 12a. The other surface of each cooler plate includes flow channels to route either hydrogen (for the anode cooler plates 20 and 24) or oxygen (for the cathode cooler plates 14 and 28) to an associated fuel cell. The bipolar plates 16 and 22 include flow channels on one surface (top or bottom surface) to route hydrogen to an associated fuel cell and flow channels on the opposing surface to route oxygen to another associated fuel cell. In this arrangement, each fuel cell may be formed in part from one bipolar plate and one cooler plate, as an example. Other fuel cell stacks have other arrangements.
To achieve optimal fuel cell performance, the components of a stack, such as the stack 10, are assembled and operated under a load or compressive force, which is also referred to as a clamping force, that is applied using a vertical press. The applied clamping force is used to compress gaskets for sealing the mating surfaces between adjacent plates to prevent leakage of the different gases and liquids in the fuel stack. In addition, the applied clamping force is used to provide a consistent pressure across the area of gas diffusion layers (GDLs) to achieve sufficient electrical conductivity between the GDLs and corresponding lands of the fuel cell plates in a stack. As illustrated in FIG. 1, one technique of applying the compressive force is by use of tie rods 24 attaching top and bottom end plates 20 and 22. The number of tie rods used may range from four to twelve. The tie rods 24 may be attached to the end plates 20 and 22 by use of washers and nuts. Typically, the end plates 20 and 22 are relatively thick and are formed of stainless steel or some other metal to provide structural support under the applied clamping force provided by the tie rods 24. However, the use of heavy stainless steel end plates and numerous tie rods, washers, and nuts lead to a relatively heavy assembly. Further, connecting the tie rods 24 to the end plates 20 and 22 to form the fuel cell assembly involves a relatively large number of steps, which may be time consuming.
Other techniques for applying a compressive force onto a fuel cell stack also exist. One such technique uses compression bands that extend around the end plates of the fuel stack assembly. However, these and other conventional clamping mechanisms are also associated with shortcomings. A need thus continues to exist for an improved apparatus and method for clamping fuel cell stack assemblies.